The present invention relates to a method for use in an apparatus for the successive discharge of individual objects of different lengths, such as items of mail for example, to control the withdrawal of each object from a stack of such objects on a conveying path in dependence on the changing position of a reference edge, which can be the leading or trailing edge, of the previously discharged object.
Prior art methods and apparatus for controlling the withdrawing procedure in such devices are disclosed in German Offenlegungsschrift [Laid-Open Patent Application] No. 24 10 145, and its counterpart U.S. Pat. No. 3,981,493, Klappenecker et al, which discloses a withdrawal device having the above-mentioned features and cooperating with a pair of continuously driven conveying rollers arranged downstream of the stack and forming the inlet for a further conveying path. This arrangement is combined with: a controllable withdrawal device which cooperates with each object to be separated from the stack and which advances that object to the effective range of the conveying rollers upon receipt of a control signal; a monitoring device which emits signals in dependence on the passage of a reference edge, i.e. the leading or trailing edge, of an object that has entered the conveying path; and a control circuit which emits a control signal, in dependence on the signals from the monitoring device, to the withdrawal device whenever the distance of the reference edge of the item being processed from the leading edge of the next object in the effective range of the withdrawal device has exceeded a given value.
In this known apparatus, the monitoring device is designed, as shown in FIG. 8 of U.S. Pat. No. 3,981,493, to emit an output signal whenever the distance between the leading edge of the object which has entered the conveying path and the leading edge of the next object which is still in the effective range of the withdrawal device has exceeded a given value B. The objects are thus discharged into the conveying path in such a way that the distances from leading edge to leading edge are identical. When separating objects having different lengths, the value for distance B must be selected so that sufficient space remains between an object with the longest length which the apparatus can handle and the next object.
Let it be assumed, for example, that during automatic processing of mail the length of the objects fluctuates between 135 mm and 240 mm and that, to assure dependable operation of the switches in a subsequent distributing machine, the gaps, or spaces, between the objects must be not less than 90 mm. To enable such gaps to be produced even with long objects, the distance B from leading edge to leading edge must therefore be given the value 240+90=330 mm, which, in order to allow for some degree of slip that might occur on a later conveying path, must be increased to 350 mm. Therefore, behind a short object, a gap of 350-135=215 mm occurs, instead of the minimum required value of 90 mm. One result is less than optimum utilization of the capacity of the subsequent sections of the system in which the objects are being conveyed.
It thus follows that it is desirable for the monitoring device to discharge the succession of objects from the stack and the control circuit so that the gaps between consecutive objects have identical lengths. The monitoring device emits an output signal when the distance of the trailing edge of the object entering the conveying path from the leading edge of the next object which is still in the effective range of the withdrawal device has exceeded a given value A. Such a device is shown, for example, in FIG. 7 of U.S. Pat. No. 3,981,493, cited above.
Theoretically, the control of the separation to produce uniform gaps should result in the highest possible conveying density. In practice, however, there is a further point which must be considered, i.e. the requirement that the above-mentioned distance from the leading edge of one object to the leading edge of the next succeeding object must not be arbitrary. Rather, in view of the mutual spacing of the light barriers disposed along the conveying path of a subsequent distributing machine and associated with an accompanying memory for the switch control, they must not fall below a certain value of, for example, 280 mm. If the control process produces gaps of identical length equal to 90 mm, the above-mentioned object length range results in leading edge distances of 135+90=225 mm to 240+90=330 mm. That means that the distance from leading edge to leading edge appearing after a short object would be too small. Therefore, to take full account of the limitations imposed by short objects, the separation process must thus be controlled in practice, for example, to produce a gap of A=280-135=145 mm for the shortest objects.
However, even this does not fully utilize the highest theoretically possible conveying output. Since nevertheless this process still produces a better utilization of the conveying output than that which produces a uniform distance B between successive leading edges, the art has been willing to be satisfied with the conveying density that can be obtained under control of the separation process which produces a uniform gap A. Separating devices used in combination with automatic mail distributing systems are presently generally controlled according to the last mentioned of the two above discussed methods.